1. Technical Field
The present invention relates to a microelectromechanical sensor having improved mechanical decoupling of sensing and driving modes. In particular, in the following description reference will be made to a gyroscope (whether uniaxial, biaxial or triaxial), which can possibly operate as an accelerometer (whether uniaxial, biaxial or triaxial).
2. Description of the Related Art
As is known, microprocessing techniques enable formation of microelectromechanical structures or systems (the so-called MEMS) within layers of semiconductor material, which have been deposited (for example, in the case of a layer of polycrystalline silicon) or grown (for example, in the case of an epitaxial layer) on top of sacrificial layers, which are removed by chemical etching. Inertial sensors, accelerometers and gyroscopes obtained with this technology are encountering an increasing success, for example in the automotive field, in inertial navigation, or in portable devices.
In particular, integrated semiconductor gyroscopes are known, which are made with MEMS technology. Gyroscopes operate according to the theorem of relative accelerations, exploiting Coriolis acceleration. When an angular velocity is imparted on a movable mass that is moving with a linear velocity, the movable mass “feels” an apparent force, referred to as Coriolis force, which causes a displacement thereof in a direction perpendicular to the direction of the linear velocity and to the axis of rotation. The movable mass is supported via springs that enable a displacement in the direction of the apparent force. According to Hooke's law, the displacement is proportional to the apparent force, and consequently, based on the displacement of the movable mass, it is possible to detect the Coriolis force and the angular velocity that has generated it. The displacement of the movable mass can, for example, be detected capacitively, by measuring, in resonance conditions, the capacitance variations caused by the movement of movable electrodes, integrally fixed to the movable mass and operatively coupled to fixed electrodes.
US2007/214883, assigned to STMicroelectronics Srl, discloses a microelectromechanical integrated sensor with a rotary driving motion, which is sensitive to pitch and roll angular velocities.
This microelectromechanical sensor includes a single driving mass, anchored to a support at a single central point and driven with rotary motion about an axis, which passes through the central point and is orthogonal to the plane of the driving mass. The rotation of the driving mass enables two mutually orthogonal components of driving velocity in the plane of the mass. At least one through opening is provided inside the driving mass, in which a sensing mass is arranged; the sensing mass is enclosed within the driving mass, suspended with respect to the substrate, and connected to the driving mass via flexible elements. The sensing mass is fixed to the driving mass during its rotary motion, and has a further degree of freedom of movement as a function of an external stress, in particular a Coriolis force acting on the sensor. The flexible elements, according to their particular construction, allow the sensing mass to perform a rotary movement of detection about an axis lying in the plane of the sensor in response to a Coriolis acceleration acting in a direction perpendicular to the plane, in a way substantially decoupled from the driving mass. The microelectromechanical structure, in addition to being compact (in so far as it envisages just one driving mass that encloses in its overall dimensions one or more sensing masses), enables with minor structural modifications, a uniaxial, biaxial or triaxial gyroscope (and/or an accelerometer, according to the electrical connections implemented) to be obtained, at the same time ensuring decoupling of the driving mass from the sensing mass during the movement of detection.
In detail, and as shown in FIG. 1, that relates to a uniaxial sensor, the microelectromechanical sensor, denoted with 1, comprises a driving structure formed by a driving mass 3 and by a driving assembly 4. The driving mass 3 has a circular geometry with radial symmetry, with a substantially planar configuration having a main extension in a plane defined by a first axis x and by a second axis y (referred to in what follows as “plane of the sensor xy”), and negligible dimension, with respect to the main extension, in a direction parallel to a third axis (referred to in what follows as “orthogonal axis z”), forming with the first and second axes x, y a set of three orthogonal axes fixed with respect to the sensor structure. In particular, the driving mass 3 has in the plane of the sensor xy substantially the shape of an annulus, and defines at the center a circular empty space 6, the center O of which coincides with the centroid and the center of symmetry of the driving mass 3. The driving mass 3 is anchored to a substrate 2 by means of a central anchorage 7 arranged at the center O, to which it is connected through elastic anchorage elements 8. For example, the elastic anchorage elements 8 depart in a crosswise configuration from the center O along a first axis of symmetry A and a second axis of symmetry B of the driving mass 3, the axes of symmetry being parallel, respectively, to the first axis x and to the second axis y. The elastic anchorage elements 8 enable a rotary movement of the driving mass 3 about a drive axis passing through the center O, parallel to the orthogonal axis z and perpendicular to the plane of the sensor xy.
The driving mass 3 has a first pair of through-openings 9a, 9b with a substantially rectangular shape elongated in a direction parallel to the second axis y, aligned in a diametric direction along the first axis of symmetry A, and set on opposite sides with respect to the empty space 6. In particular, the direction of alignment of the through-openings 9a, 9b corresponds to a direction of detection of the microelectromechanical sensor 1 (in the case represented in the figure, coinciding with the first axis x).
The driving assembly 4 comprises a plurality of driven arms 10 (for example, eight in number), extending externally from the driving mass 3 in a radial direction and spaced apart at a same angular distance, and a plurality of first and second driving arms 12a, 12b, extending parallel to, and on opposite sides of, respective driven arms 10 and anchored to the substrate via respective anchorages. Each driven arm 10 carries a plurality of first electrodes 13, extending in a direction perpendicular to, and on either side of, the driven arm. Furthermore, each of the first and second driving arms 12a, 12b carries respective second electrodes 14a, 14b, extending towards the respective driven arm 10 and comb-fingered to the corresponding first electrodes 13. The first driving arms 12a are all arranged on the same side of the respective driven arms 10 and are all biased at a first voltage. Likewise, the second driving arms 12b are all arranged on the opposite side of the respective driven arms 10, and are all biased at a second voltage. In a per se known manner which is not described in detail, a driving circuit is connected to the second electrodes 14a, 14b so as to apply the first and second voltages and determine, by means of mutual and alternating attraction of the electrodes, an oscillatory rotary motion of the driving mass 3 about the drive axis, at a given oscillation frequency.
The microelectromechanical sensor 1 further comprises a first pair of acceleration sensors with axis parallel to the orthogonal axis z, and in particular a first pair of first sensing masses 16a, 16b, each positioned in a respective one of the through-openings 9a, 9b, so as to be completely enclosed and contained within the overall dimensions of the driving mass 3 in the plane of the sensor xy. The first sensing masses 16a, 16b have a generally rectangular shape matching the shape of the respective through opening 9a, 9b, and are formed by a first rectangular portion 17, which is wider, and by a second rectangular portion 18, which is narrower (along the first axis x), connected by a connecting portion 19, which is shorter (in a direction parallel to the second axis y) than the first and second rectangular portions. Each first sensing mass 16a, 16b has a centroid G located within the corresponding first rectangular portion 17, and is supported by a pair of elastic supporting elements 20. The elastic supporting elements 20 are connected to the connecting portion 19, and extend towards the driving mass 3, in a direction parallel to the second axis y. In other words, the elastic supporting elements 20 extend within recesses 21 provided at opposite sides of the sensing masses 16a, 16b. The elastic supporting elements 20 extend at a distance from the centroid G of the respective first sensing mass 16a, 16b, and form torsional springs that are rigid for the rotary motion of the driving mass 3, and also enable rotation of the sensing masses about an axis of rotation parallel to the second axis y and lying in the plane of the sensor xy (and, consequently, their movement out of the plane of the sensor xy).
A pair of first and second detection electrodes 22, 23 is arranged underneath the first and second rectangular portions 17, 18 of each one of the first sensing masses 16a-16b; for example the detection electrodes 22, 23 are constituted by regions of polycrystalline silicon formed on the substrate 2, having equal dimensions substantially corresponding to those of the second (and smaller) rectangular portion 18. The first and second detection electrodes 22, 23 are separated, respectively from the first and second rectangular portions 17, 18, by an air gap, and are connected to a read circuit. The first and second detection electrodes 22, 23 hence form, together with the first and second rectangular portions 17, 18 respective detection capacitors.
In use, the microelectromechanical sensor 1 is able to operate as a gyroscope, designed to detect an angular velocity {right arrow over (Ω)}x (in FIG. 1 assumed as being counterclockwise), about the first axis x.
On the hypothesis of small displacements of the first sensing masses 16a-16b and of small rotations of the driving mass 3, the rotary movement of the driving mass 3 and of the first sensing masses 16a-16b about the drive axis can be represented by a driving-velocity vector {right arrow over (v)}a, tangential to the circumference that describes the driving trajectory.
In particular, the rotary motion about the first axis x at the angular velocity {right arrow over (Ω)}x determines a force acting on the entire structure, known as Coriolis force (designated by {right arrow over (F)}c). In particular, the Coriolis force {right arrow over (F)}c is proportional to the vector product between the angular velocity {right arrow over (Ω)}x and the driving velocity {right arrow over (v)}a, and is hence directed along the orthogonal axis z, is zero in the points where the driving velocity {right arrow over (v)}a is parallel to the first axis x, and, in the points where it does not go to zero, it is directly proportional to the driving velocity {right arrow over (v)}a, and consequently it increases with the distance from the center O. Over the entire structure, considered as a single rigid body, it is hence possible to identify a distribution of Coriolis forces that vary as the distance from the center O varies. The resultants of the Coriolis forces {right arrow over (F)}c acting on the first sensing masses 16a, 16b at the corresponding centroid G, cause rotation of the sensing masses, which move out of the plane of the sensor xy, about an axis parallel to the second axis y and passing through the first elastic supporting elements 20. This movement is allowed by the torsion of the first elastic supporting elements 20. Instead, the configuration of the elastic anchorage elements 8 is such as to inhibit, at least to a first approximation (see the following discussion), movement of the driving mass 3 out of the plane of the sensor xy, thus allowing decoupling of the motion of detection of the first sensing masses from the driving motion. The displacement of the first sensing masses 16a, 16b out of the plane of the sensor xy causes a differential capacitive variation of the detection capacitors, the value of which is proportional to the angular velocity {right arrow over (Ω)}x, which can hence be determined in a per-se known manner via a purposely provided read circuit. In particular, since the reading scheme is differential, the presence of a pair of first sensing masses enables automatic rejection of spurious linear accelerations along the orthogonal axis z. These accelerations, in fact, cause a variation in the same direction of the detection capacitors, which is cancelled by the differential reading (on the contrary, the same structure can be operated as an accelerometer for detecting the accelerations along the orthogonal axis z, simply by modifying the electrical connections between the sensing masses and electrodes). The presence of the central anchorage also enables rejection of spurious linear accelerations along the axes x and y, given that the arrangement of elastic anchorage elements 8 is extremely rigid in these directions, and does not enable displacement of the sensing masses. Furthermore, the described structure is able to mechanically reject spurious angular acceleration about the orthogonal axis z, since the frequency response of the sensor can be modeled as a very selective filter.
Although it is advantageous with respect to traditional gyroscope structures, the Applicant has realized that the described microelectromechanical sensor is not optimized, in particular with respect to the decoupling between the driving and sensing modes of operation.
In detail, the Applicant has realized that flaws in the manufacturing process or improper choices in the structure geometry (e.g. a thickness too small with respect to the dimensions in the plane of the sensor xy, or an improper shape of the elastic elements) may result in the microelectromechanical structure having an improper ratio between the stiffness in the orthogonal direction z and the stiffness in the plane of the sensor xy. In particular, the driving mass 3 could have an insufficient stiffness in the orthogonal direction z, so that application of the Coriolis force Fc would lead to oscillations movement outside of the plane of the sensor xy not only by the sensing masses (as desired) but also by the same driving mass (contrary to the expected operation). In other words, the decoupling between the driving and sensing movements could be impaired.
FIG. 2 shows a situation in which the stiffness of the structure in the orthogonal direction z (provided by the elastic anchorage elements 8 connecting the driving mass 3 to the central anchorage 7) is not sufficient to avoid undesired movements of the driving mass 3 outside the plane of the sensor xy, following application of the Coriolis force Fc.
The lack of a perfect decoupling between the driving and sensing movements entails a number of disadvantages in the microelectromechanical sensor.
Firstly, any non-ideality in the driving arrangement affects also the sensing arrangement, and vice versa.
Secondly, during sensing operations, the driving movement is altered, mainly due to the variation in the facing area of the driving electrodes (first electrodes 13 and corresponding second electrodes 14a, 14b), because of the movement of the driving mass 3 outside of the plane of the sensor xy. Indeed, the Coriolis force Fc is a function of the tangential driving velocity {right arrow over (v)}a, according to the expression:
      F    c    =            2      ·      m      ·              Ω        ->              ×                  v        ->            a      wherein m is the mass of the sensing mass, {right arrow over (Ω)} is the angular velocity that is to be detected (e.g. the angular velocity {right arrow over (Ω)}x) and {right arrow over (v)}a is the driving velocity at the application point of the Coriolis force Fc. A variation of the driving velocity {right arrow over (v)}a due to a different facing area between the electrodes causes a corresponding variation of the Coriolis force Fc and a variation in the output gain of the sensor. As a result, an undesired variation of the overall sensitivity of the microelectromechanical sensor 1 may occur.
Finally, a structure that is compliant (to a certain degree) outside the plane of the sensor xy is inevitably more affected to shock directed along the orthogonal direction z.